Fire detection systems and methods are employed in most commercial and industrial environments, as well as in shipboard environments that include commercial and naval maritime vessels. Conventional systems typically have disadvantages that include high false alarm rates, poor response times, and overall sensitivity problems. Although it is desirable to have a system that promptly and accurately responds to a fire occurrence, it as also necessary to provide one that is not activated by spurious events, especially if the space contains high-valued, sensitive materials or the release of a fire suppressant is involved.
Economical fire and smoke detectors are used in residential and commercial security, with a principal goal of high sensitivity and accuracy. The sensors are typically point detectors, such as photoionization, photoelectron, and heat sensors. Line detectors such as beam smoke detectors also have been deployed in warehouse-type compartments. These sensors rely on diffusion, the transport of smoke, heat or gases to operate. Some recently proposed systems incorporate different types of point detectors into a neural network, which may achieve better accuracy and response times than individual single sensors alone but lack the faster response time possible with remote sensing, e.g., optical detection. Remote sensing methods do not rely on effluent diffusion to operate.
An optical fire detector (OFD) can monitor a space remotely, i.e. without having to rely on diffusion, and in principle can respond faster than point detectors. A drawback is that it is most effective with a direct line of sight (LOS) to the source, therefore a single detector may not provide effective coverage for a monitored space. Commercial OFDs typically employ a single/multiple detection approach, sensing emitted radiation in narrow spectral regions where flames emit strongly. Most OFDs include mid infrared (MIR) detection, particularly at 4.3 μm, where there is strong emission from carbon dioxide. OFDs are effective at monitoring a wide area, but these are primarily flame detectors and not very sensitive to smoldering fires. These are also not effective for detecting hot objects or reflected light. This is due to the sensitivity trade-offs necessary to keep the false alarm rates for the OFDs low. Other approaches such as thermal imaging using a mid infrared camera are generally too expensive for most applications.
Video Image Detection Systems (VIDS), such as the Fire Sentry VSD-8, are a recent development. These use video cameras operating in the visible range and analyze the images using machine vision. These are most effective at identifying smoke and less successful at detecting flame, particularly for small, emergent source (either directly or indirectly viewed, or hot objects). Hybrid or combined systems incorporating VIDS have been developed in which additional functionality is achieved using radiation emission sensor-based systems for improved response times, better false alarm resistance, and better coverage of the area with a minimum number of sensors, especially for obstructed or cluttered spaces
U.S. Pat. No. 5,937,077, Chan et al., describes an imaging flame detection system that uses a charge coupled device (CCD) array sensitive in the IR range to detect IR images indicative of a fire. A narrow band IR filter centered at 1,140 nm is provided to remove false alarms resulting from the background image. Its disadvantages include that it does not sense in the visible or near-IR region, and it does not disclose the capability to detect reflected or indirect radiation from a fire, limiting its effectiveness, especially regarding the goal of maximum area coverage for spaces that are cluttered in which many areas cannot be monitored via line of sight detection using a single sensor unit. U.S. Pat. No. 6,111,511, Sivathanu et al., describes photodiode detector reflected radiation detection capability but does not describe an image detection capability. The lack of an imaging capability limits its usefulness in discriminating between real fires and false alarms and in identifying the nature of the source emission, which is presumably hot. This approach is more suitable for background-free environments, e.g., for monitoring forest fires, tunnels, or aircraft cargo bays, but is not as robust for indoor environments or those with a significant background variation difficult to discriminate against.
U.S. Pat. No. 6,529,132, G. Boucourt, discloses a device for monitoring an enclosure, such as an aircraft hold, that includes a CCD sensor-based camera, sensitive in the range of 0.4 μm to 1.1 μm, fitted with an infrared filter filtering between 0.4 μm and 0.8 μm. The device is positioned to detect the shifting of contents in the hold as well as to detect direct radiation. It does not disclose a method of optimally positioning the device to detect obstructed views of fires by sensing indirect fire radiation or suggest a manner in which the device would be installed in a ship space. The disclosed motion detection method is limited to image scenes with little or no dynamic motion.
It is desirable to provide a fire detection method that can detect images and that can also sense indirect radiation, including reflected and scattered radiation.